Evaluating the impacts of food quality on bluegill growth rate during early ontogeny

Introduction • Energy transfer through a food chain can be

influenced by food quality of primary producers (carbon:nutrient ratios)1,2,3.

• Low quality at the producer stage limits the energy transfer to top predators (Fig 1)1,2.

• Light availability and nutrient input influence the amount of carbon that can be produced in primary producers, thereby influencing C:nutrient ratios (food quality)4.

• High light results in high C:nutrient ratios which is poor food quality for top predators, whereas high nutrient input decreases C:nutrient ratios (Fig. 1)

• Bluegill have high P requirements so they are more likely to be susceptible to decreased growth from P limitation5,6

• Larval bluegill may be especially vulnerable to P-limitation as they build bones

• RNA content is a useful tool because it is closely linked to growth rate and is also a P-rich molecule

Objective: To evaluate the RNA content in larval and

juvenile bluegill raised under different light and nutrient supply conditions.

Methods• Experiment conducted in 24 5,000-L mesocosms at

Miami University’s Ecology Research Center

• Manipulated light and nutrients in a factorial design

• High (HL) vs Low (LL) Light – accomplished by covering mesocosms with shade cloth

• High (HN) vs Low(LN) nutrients – added nutrients (N and P) to HN treatments

• Half of the tanks were drained at the larval bluegill stage (1 week into experiment) and the rest were drained at the juvenile bluegill stage (6 weeks into experiment)

• 3 fish were randomly collected from each tank and preserved with RNAlater® (Life Technologies)

• RNA was isolated from whole larval and juvenile fish with Promega SV Total RNA Isolation kits

• RNA content was quantified using a Thermo Scientific NanoDrop spectrophotometer

• Because RNA content is related to mass (Fig. 3), data was mass-normalized before using 2-way ANOVA to evaluate effects of light and nutrients

Conclusions• Prediction 1 was partially supported

• Bluegill RNA content decreased with age only in the HLHN treatment

• Prediction 2 was not supported• No effects of food quality on RNA content

• High RNA content in the HLHN treatment suggests that larval fish are more dependent on food quantity rather than food quality

Future Research• We plan to measure DNA of all fish to determine

RNA:DNA ratio

Acknowledgements: We would like to thank Rodney Kolb and the ERC staff for their help setting up the mesocosm experiment, Andor Kiss and the Miami University Center for Bioinformatics and Functional Genomics for assistance with sample processing, and the Williamson lab for loaning us equipment. This project was funded by NSF grant DEB-0949500 to M. González and M. Vanni, a Miami University biology department Academic Challenge grant to A. Rock, and a Miami University DUOS award to E. Barnes and A. Rock.

Predictions 1. RNA content should decrease with age (Fig. 2)

• Growth rate: larval > juvenile • Growth rate typically decreases as organisms age

2. RNA content will be highest under low light/high nutrients • Food quality will be highest under low light/high

nutrient supply (Fig. 2)

Results• RNA content was negatively correlated with fish mass

as expected (Fig. 3)

• Larval HLHN treatments had significantly higher RNA content (μg/mg fish) than the other treatments (Fig. 4)

• Light (p=0.0004), Nutrients (p=0.003), and Light*Nutrients (p=0.0001) effects were all significant

• In the juvenile stage, RNA content was highest in the LLLN treatment and lowest in the HLLN treatment (Fig. 4)

• Treatment differences were not significant

• Trends in RNA content were very similar to trends in relative growth rate (Figs 4, 5)

Emma Barnes, Amber Rock, Luke Ginger, Michael Vanni, María González

Miami University, Oxford OH

Figure 4. Relationship between food quality, ontogeny (larval and juvenile stages), and RNA content (mean ± SE) of bluegill. Dotted horizontal line indicates average initial RNA content.

Figure 3. RNA content (μg/mg fish) vs. individual fish mass of larval and juvenile fish. Dotted horizontal line indicates average initial RNA content, dotted vertical line indicates average initial mass.

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Figure 2. Predicted relationships between food quality, ontogeny (larval and juvenile stages), and RNA content of bluegill.

Figure 1. Light and nutrient supply influences primary producer stoichiometry, and these effects can carry up the food chain to affect herbivores and carnivores

Figure 5. Relationship between food quality, ontogeny (larval and juvenile stages), and relative growth rate of bluegill.

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References:1. Dickman, E., Newell, J., Gonzalez, M. & Vanni, M. J. Light, nutrients, and food-chain length constrain planktonic energy transfer efficiency across multiple trophic levels. Proc. Natl. Acad. Sci. U. S. A. 105, 18408–18412 (2008).2. Boersma, M. et al. Nutritional Limitation Travels up the Food Chain. Int. Rev. Hydrobiol. 93, 479–488 (2008).3. Sterner, R. W. & Elser, J. J. Ecological stoichiometry: the biology of elements from molecules to the biosphere. (Princeton University Press, 2002).4. Sterner, R. W., Elser, J. J., Fee, E. J., Guildford, S. J. & Chrzanowski, T. H. The light: nutrient ratio in lakes: the balance of energy and materials affects ecosystem structure and process. Am. Nat. 150, 663–684 (1997)..5. Sterner, R. & George, N. Carbon, nitrogen, and phosphorus stoichiometry of cyprinid fishes. Ecology 81, 127–140 (2000).6. Vanni, M. J., Flecker, A. S., Hood, J. M. & Headworth, J. L. Stoichiometry of nutrient recycling by vertebrates in a tropical stream: linking species identity and ecosystem processes. Ecol. Lett. 5, 285–293 (2002).